One of the persistent challenges faced in the development of semiconductor technology is the desire to increase the density of circuit elements and interconnections on substrates without introducing spurious interactions between them. Unwanted interactions are typically prevented by providing gaps or trenches that are filled with electrically insulative material to isolate the elements both physically and electrically. As circuit densities increase, however, the widths of these gaps decrease, increasing their aspect ratios and making it progressively more difficult to fill the gaps without leaving voids. The formation of voids when the gap is not filled completely is undesirable because they may adversely affect operation of the completed device, such as by trapping impurities within the insulative material.
Common techniques that are used in such gapfill applications include chemical-vapor deposition (“CVD”) techniques. Conventional thermal CVD processes supply reactive gases to the substrate surface where heat-induced chemical reactions take place to produce a desired film. Plasma-enhanced CVD (“PECVD”) techniques promote excitation and/or dissociation of the reactant gases by the application of radio-frequency (“RF”) energy to a reaction zone near the substrate surface, thereby creating a plasma. The high reactivity of the species in the plasma reduces the energy required for a chemical reaction to take place, and thus lowers the temperature required for such CVD processes when compared with conventional thermal CVD processes. These advantages may be further exploited by high-density-plasma (“HDP”) CVD techniques, in which a dense plasma is formed at low vacuum pressures so that the plasma species are even more reactive. While each of these techniques falls broadly under the umbrella of “CVD techniques,” each of them has characteristic properties that make them more or less suitable for certain specific applications.
In some instances where gaps have a large aspect ratio and narrow width, gaps have been filled with thermal CVD techniques using a “dep/etch/dep” process by sequentially depositing material, etching some of it back, and depositing additional material. The etching step acts to reshape the partially filled gap, opening it so that more material can be deposited before it closes up and leaves an interior gap. Such dep/etch/dep processes have also been used with PECVD techniques, but some thermal and PECVD techniques are still unable to fill gaps having very large aspect ratios even by cycling deposition and etching steps.
Cycling of deposition and etching steps was traditionally view by those of skill in the art as inutile in the context of HDP-CVD processes. This view resulted from the fact that, very much unlike PECVD processes, the high density of ionic species in the plasma during HDP-CVD processes causes there to be sputtering of a film even while it is being deposited. This simultaneous sputtering and deposition of material during a deposition process tends to keep the gap open during deposition, and was therefore believed to render a separate intermediate etching superfluous. This prevailing view proved to be partially correct in that higher-aspect-ratio gaps could be filled using an HDP-CVD process than could be filled even with a PECVD dep/etch/dep process. Nevertheless, in U.S. Pat. No. 6,194,038, filed Mar. 20, 1998 by Kent Rossman, the unexpected result was demonstrated that gapfill could be improved even further by using a dep/etch/dep process under certain HDP-CVD process conditions. This result was later confirmed in U.S. Pat. No. 6,030,881, filed May 5, 1998 by George D. Papasouliotis et al.
It turns out that even with the combined deposition and sputtering of HDP-CVD processes, the gap still tends to close when narrow-width high-aspect-ratio structures are filled. The use dep/etch/dep techniques in HDP processes have thus followed the traditional path of dep/etch/dep techniques by depositing sufficient material to partially fill the gap, followed by etching to reshape the gap for further deposition. The steady reduction in feature sizes is currently reaching the stage where the utility of such techniques is approaching its limit. This is particularly true for certain structure geometries, such as shallow-trench-isolation (“STI”) structures, that may have both narrow gaps and open regions. As the gap has become more aggressive, increasing number of cycles have been found to be necessary to fill the gaps, with the consequence that each deposition step fills less of the gap by depositing a smaller amount of material. A correspondingly small amount of material is thus deposited in open regions, with the result that the subsequent etching step tends to remove too much of the thin layer of material in the open regions, damaging the underlying structure.
There is accordingly a remaining need in the art to improve HDP-CVD dep/etch/dep processes to accommodate narrow high-aspect-ratio structures.